Build failing at GHOST_DropTargetX11.cpp

A few weeks ago my build started to fail at GHOST_DropTargetX11.cpp

Due to the file not being changed for some time, I wasn’t able to pick the change that broke it but I need to include stdio.h for strncpy() and sscanf()

While I don’t expect an issue including stdio.h on every platform I am on FreeBSD so it could be wrapped in if defined(__FreeBSD__) if there is a desire to limit it.

Thanks, fix committed in